This invention relates to magnetic tape transports and more particularly to a tape transport system and method adapted for use in high density data storage of either digital or analog data. As used herein tape transports and tape recorder will be used interchangeably and include tape players. Tape recorders and similar devices in which a spool of tape is moved from one to a proximate reel past a record playback magnetic head which either impresses magnetic signals on or receives magnetic signals from the tape. Movement of the tape strip between the reels has heretofore been achieved using a constant speed linear movement mechanisms using either a capstan drive or a reel drive. While the capstan drive is relatively easy to implement and works well in constant speed applications it is not very well adapted in variable high speed high information density uses, particularly where constant search and locate functions are demanded, as in data retrieval.
U.S. Pat. No. 4,163,532 issued Aug. 7, 1979, to Yoshikai Sakai discloses a transport adapted for data cassette use employing a variable speed motor drive, a velocity sensing take-up drive governed by speed sensing wheel moved by the tape and the dynamic supply drive. The latter forming a velocity tack loop by which the absolute speed of movement of the tape is controlled. A tape tension control is also provided by energizing the supply side of motor, however, the Sakai system is not adapted to absolute control of the tape speed as a function of an accurate timing thereof based on the origin of the signal being recorded, nor is it adapted to control the tension of the tape so as to maintain a constant value.
A phase-lock loop take-up drive has been known for speed control of a capstan driven system, Horak et al, U.S. Pat. Nos. 4,218,713 issued Aug. 19, 1980 and Rotter et al, 4,123,773 issued Oct. 30, 1978. A recorded signal on the tape has also been used as a tape drive speed reference, see Hamilton U.S. Pat. No. 3,596,005 issued July 25, 1971 While these prior art patents do provide a solution to the speed control problem in magnetic tape recording they fall short of achieving a general solution in that their resulting access time is limited, in the case of capstan systems and their speed accuracy depends on the speed accuracy of the original recording itself or on the speed accuracy of a reference track. Furthermore, none of the prior art referenced patents discloses a sufficiently accurate dynamic tension system controlled by a function corresponding to a measurement of tape tension itself. There is therefor a need for an improved transport system for magnetic recording and playback.
In general, it is an object of the present invention to provide an improved tape transport system and method which will overcome the limitations and disadvantages.
A further object of the invention is to provide the tape transport of the above character which achieves a high data packing density which, can be readily operated in fast scan modes and possesses high data recovery accuracy while in such modes.
A further object of the invention is to provide a tape transport system of the above character in which tape handling during scan and search modes is accomplished using a tachometer reference phase-lock loop and in which tape handling during signal processing is accomplished using a tape to signal accurate reference, the accuracy of which depends solely on the combined accuracy of the tape reference signal and the phase-lock reference, independently of the absolute accuracy of the tape transport movement, tape elasticity, and other variables.
Another object of the invention is to provide a tape transport system of the above character in which tape acceleration and deceleration circuits provide tension control which electronically sums the tension in the tape and acts to maintain that tension constant at all times, except for rapid deceleration.
Another object of the invention is to provide tape transport system of the above character which is completely bi-directional and operational.
The present invention relates additionally to medical electronics applications for which purpose it was first developed. In that connection reference is made to U.S. Pat. Nos. 3,215,136 issued Nov. 2, 1965 to Holter, to 4,073,011 (now Re. 29,921) issued Feb. 7, 1978; 4,123,785 issued Oct. 31, 1978 to Cherry et al to 4,127,571 issued June 5, 1979 to Shu, and to U.S. patent application Ser. No. 192,192 filed Sept. 30, 1980, in the names of Anderson, Cherry, Ripley and Tanaka, all of which are assigned to the same assignee as the present application.
In recent years considerable progress has been made in recording and analyzing of electrocardiac signals obtained from a patient, particularly an ambulatory patient. Thus, in the Holter technique, the patient is provided with a small portable tape recorder on which his cardiac signals are recorded over an extended period of time as the patient goes about his daily activities. Such signals are usually continuously recorded over intervals up to 24 hours on a single tape, using tape speeds as low as 1/16 of an inch per second. Since it would be prohibitively expensive to examine an entire 24 hour recording at real time speed, such tapes are normally scanned at much higher speeds on special high speed scanners, such as that described in U.S. Pat. No. 4,006,737 entitled "Electrocardiographic Computer" issued Feb. 8, 1977 to Cherry as modified by such improvements as in U.S. Pat. No. 4,157,571. Thus, the playback scanner is provided with a capability of displaying a signal at various speeds and in addition the ability to freeze displayed action for extended study. The electrocardiac signals have a significant frequency component in the lower audio range so that the design of playback heads capable of satisfactory operation at high speed and at real time playback speeds is difficult. As a solution, scanners have been developed which permit a segment of the signal on the magnetic tape to be frozen in a display and advanced or reversed in various slow speeds without relying on particular magnetic head considerations. As will be understood from review of the referenced patents and patent application, the ability to start and to stop and to proceed at varying scan speeds is a requisite of these recording techniques. In addition, the high packing density of ECG signals, within 0.010 of an inch, and requisite time accuracy of the resultant signal in display time necessitates a very accurate recording and playback system. It will be further understood that such a tape transport system must be useable under circumstances providing for ambulatory movement of the patient in which the recording device is both battery operated and subject to constant movement and acceleration with the patient's body and thus, subject to speed fluctuations both for that reason and because of battery powered operation. As mentioned in scanning such tapes the stop, reload data, and scan at speed signals must be implemented over the range from real time to 240 times the real time and higher. That coupled with the position and time accuracy required to single out an ECG complex on the tape requires tolerances heretofore unavailable. By way of example, even tape stretch during acceleration and deceleration can amount to a significant movement of the position of an ECG on the tape.
Accordingly, it is desired to provide a tape transport system in the present invention in which positional accuracy of recorded signal segments on the tape is no longer relied upon and tape stretch will not affect, within limits, the useful accuracy of the tape transport of this invention.
In the present invention absolute positional accuracy is not required. In the take-up reel drive circuits, as set forth herein, a phase-lock loop drive is used and referenced to a tack signal drive from tape movement during periods of speed change and stop mode. Speed change is affected by a ramp generator and VCO which shift speeds smoothly and gradually after which, when at speed, the phase-lock loop take-up drive is shifted to a reference signal of accurate time which is recorded on the tape simultaneously with the recording of the data or information signal. By so doing any speed variations and tape stretch becomes removeable during playback by continuing speed regulation of the tape to maintain the time signal constant against the crystal control time signal of the phase-lock loop circuit. In addition to the foregoing current a second servo loop is employed for controlling the supply reel drive motor. This loop is controlled in part by a current sensing circuit which senses tape tension electronically and fixes the tape tension as a constant input drive to the supply reel. Other supply current drives include a constant braking bias signal and a compensation feedback signal derived from a portion of the take-up reel drive. In addition, a special deceleration current is employed for rapid stopping of the system.
Accordingly, it is a further object of the invention to provide a tape transport system and method of the above character which is particularly adapted for recording ECG signals on magnetic tape and for playing back the same with a high degree of time accuracy irrespective of the positional accuracy of the tape during various record and playback operations.
A further object of the invention is to provide a tape transport system and method of the above character in which the tape tension is computed in a servo loop for controlling supply reel movement in relation to take-up reel movement, such that the tape tension tends towards a constant value.